Structural Regulation And Performance Of Silicon/Graphite Composite Anode Materials For Lithium-Ion Batteries

With the rapid development of electronic equipment and related fields of new energy electric vehicles,higher requirements are put forward for energy density and cycle life of the lithium-ion batteries.Graphite has good electrical conductivity,and is widely used for lithium-ion battery anode materials.However,the capacity of traditional graphite anodes has been close to its theoretical limit,which cannot meet the growing needs of long-distance electric vehicles.Among the emerging anode materials,silicon（Si）and its oxides（SiO_x）are considered to be the most prominent candidate anode materials due to their high theoretical capacity,low electrochemical potential（about 0.2-0.4 V）,and abundant crustal reserves.However,the conductivity of silicon-based materials is poor,and the silicon-based anodes have a serious volume expansion effect during alloying with lithium,which will cause material cracking,pulverization and other problems,which seriously limits its application.In view of the above problems,a series of silicon/graphite and silicon oxide/graphite composites with special structure and uniform stability were prepared through reasonable structural design regulation,surface modification and carbon coating by using graphite as matrix material and the synergistic effect of silicon and graphite.On the basis of material characterization,their lithium storage properties were systematically studied.The main research contents and results are as follows:（1）The natural superfine graphite,melamine and polyvinylpyrrolidone（PVP）were used as raw materials to prepare nitrogen-doped ultrafine graphite microspheres with a certain buffer structure by spray drying.Graphite flakes are randomly oriented inside spherical particles and connected to each other,with isotropic properties,which can effectively reduce electron transfer resistance.The morphology and structure of the microspheres and their electrochemical properties were regulated by optimizing the proportion of binders.Coating the microspheres with amorphous carbon can significantly reduce the specific surface area of the material,enhance the connection between the graphite flakes,and improve the structural stability of the material.By blocking the direct contact between the organic electrolyte and the inner ultrafine graphite,the consumption of the electrolyte and the occurrence of side reactions are reduced.Thereby,the initial coulombic efficiency of the material is improved,and the cycle stability of the material is improved.The carbon-coated hollow graphite microspheres still have a reversible capacity of 304 m Ah g^-1 after 500 cycles at a current density of 1 Ag^-1,and the capacity retention is about 99%.（2）Nano-silicon particles were introduced into the graphite microspheres using a scalable spray-drying method.In the prepared silicon/superfine graphite composite microspheres,nano-Si is uniformly dispersed in the superfine graphite framework,and the amorphous carbon generated by carbonization of organic matter acts as a conductive binder,connecting silicon and superfine graphite,to improve the conductivity of the material.By introducing sodium alginate into the microsphere system,the conductive carbon network structure formed by carbonization（SAC）acts as the link between the active materials,which can significantly improve the structural stability of the material.After ultrasonic treatment in aqueous solution for 30 min,the complete spherical structure could still be maintained,and the morphology was almost unchanged.In addition,during the high-temperature heat treatment of sodium alginate,the surface Si particles was partially converted into SiO_x to form a SiO_x@Si structure.Although the specific capacity of the material was slightly reduced,the volume expansion effect of silicon was significantly inhibited,and the stability of the charge and discharge process of the anode material was improved.At the current density of 1 Ag^-1,the capacity retention rate of SiO_x@Si/SAC@G microspheres after 300 cycles was 90.8%,and that after 500 cycles was 78.5%,showing good cycle stability.（3）The spherical graphite and nano-silicon were used as raw materials,mildly-expanded graphite with a uniform expansion structure was obtained by using concentrated sulfuric acid and hydrogen peroxide to perform mild-expansion modification on spherical graphite at room temperature.The surface of the graphite had oxygen-containing functional groups such as hydroxyl and carboxyl groups,and the surface of silicon particles was modified by a large number of hydroxyl groups.A strong bonding occurs between the surface-modified silicon and graphite,so that the silicon particles are adsorbed on the surface and lamellar space of the mildly-expanded graphite.Through the in-situ polymerization of dopamine on the silicon/mildly-expanded graphite surface to form polydopamine,a large number of hydroxyl groups and amino groups on the polydopamine surface generated hydrogen bonds with the functional groups on the mildly-expanded graphite and the silicon surface,so that the silicon particles are evenly distributed in the expansion space of mildly-expanded graphite,and ensure a certain bonding strength.After heat treatment,polydopamine was carbonized into amorphous carbon to ensure that the silicon nanoparticles avoided direct contact with the electrolyte,and the silicon particles were further fixed on the mildly-expanded graphite substrate to form buffer space for volume effects of Si.At the current density of 0.5 Ag^-1,the capacity retention rate of the material after 400 cycles was 75.2%.（4）The surface modification of silicon and silicon oxide（SiO）with poly（diallyldimethylammonium chloride）（PDDA）can make the positively charged,and then they have a strong electrostatic interaction with negatively charged mildly-expanded graphite in aqueous solution,so that the silicon particles are evenly and tightly adsorbed on the mildly-expanded graphite substate.Through the amorphous carbon coating,the connection between silicon and graphite was strengthened,and the stability of the material structure was enhanced.The amorphous carbon layer and mildly-expanded graphite together forms a buffer space,which effectively alleviated the volume expansion and pulverization of silicon particles and improved the cycle stability of the material.The initial coulombic efficiency of C@p-Si/ESG composites was 85.7%.At a high current density of 1 Ag^-1,the capacity retention rate was 82.3%after 300 cycles,and 70.8%after500 cycles.In addition,the silicon oxide/mildly-expanded graphite composite（C@p-SiO/ESG）prepared by the electrostatic assembly method,after being treated at a high temperature of 950°C,the SiO in the material underwent a disproportionation reaction to generate silicon dioxide and silicon,which reduced the specific capacity of the material,but the capacity retention was improved due to a lower expansion effect.